Several laboratories have demonstrated the potential of immunotoxins (anti-tumor monoclonal antibody-toxin conjugates) for anti-tumor therapy. A major problem with this technique, in addition to that of obtaining an appropriate monoclonal antibody, is the relatively short circulating half-life of immunotoxin and the potential of immunotoxin to induce an immunological response. The objective of this investigation is to evaluate the potential of using chemical modification with the water soluble polymer polyethylene glycol to increase circulating half-life and decrease immunogenicity of immunotoxin while maintaining both antibody and toxin activity. Similar studies have proven successful with several enzymes. (HI)